[geos-devel] How does "intersects" work?

Martin Davis mbdavis at refractions.net
Fri Jan 18 15:08:48 EST 2008

Let me try...

The two input geometries are decomposed into labelled topology graphs 
(GeometryGraphs).  The labels are on the nodes and edges of the graphs, 
and contain full information about the topology of the node/edge in the 
points/lines/polygons of the original geometry.  These graphs are 
merged.  The resulting relationship (Intersection Matrix) is determined 
by the merged label information at the nodes of the merged graph. The 
labelling of each node and its incident edges is inspected, and the 
topological relationship information it contributes is added to the 
overall IM. At the end of this process the IM has been completely 

HTH - Martin

Meek, Allan (Contractor) wrote:
> Is there a brief explanation of how the "intersects" function in
> GEOS/JTS works? I've traced the code down to RelateComputer::computeIM,
> but the code comments fly over my head. :)
> Thanks,
> Allan
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Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022

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